Genes lie all the time, and have been doing so for about 800 million years.
No. Women have a tendency to lie to men and tell them that they're the father of the brat, not the next door neighbor and not the postman.
But genetic testing of paternity is 99.9% accurate.
(For that matter, men have a tendency to lie to women and tell them that it's just a weird coincidence that the child of their sister or best friend looks just like the man who's swearing he would never, never, ever do such a thing.)
Nope, people lie, but genes don't.